There are special payment processors catering to porn/etc, but the problem is that they require extremely large collateral and have other unusual requirements that are untenable to new businesses. Most probably they won't even talk to a startup (I tried, didn't get a reply). There's a reason why startups like OnlyFans used normal payment processors and tried to convince them they're not porn instead of beginning with these specialized ones.

Edit: BTW, a large portion of seemingly independent porn sites are actually run by only few companies. Especially the Xvideos corporation.

So we should pivot to building an open source payment processor for porn?

A payment processor is an entity that handles bureaucracy. It's not a piece of software you can run and forget about. It is many people working around the clock taking phone calls.

I should have been clearer and said "software to aid in running a payment processor" but I used "payment processor" as a shorthand. Just as "open-source Amazon" is shorthand for "software to aid in running an entity like Amazon."